In the U.S., you have to be atleast 18 years of age to sign any type of contract and a gym membership is a contract. However, your parents can sign with or for you.
Membership varies, your choice from 3 months and up; of course, you decide depending upon how much of a commitment you feel comfortable with. Always remember, if you decide on a long term membership...
Related ads:
Shopping for Health deals?
Find Coupons and Promo Codes on Coupons by Answers.com